Rechargeable batteries are essential components for consumer electronics, electric vehicle, and large grid energy storage. The state-of-the-art lithium ion batteries have high energy density and power density; however, their limitations lie in the high cost, low natural abundance of lithium, and safety issues related to dendrite formation. Efforts continue in the search for alternative electrode and electrolyte materials that are environmentally benign and of lower cost.
Magnesium is one of the most abundant elements on earth, and is an attractive electrode material with a high theoretical specific capacity of 2205 Ah/kg and a high theoretical energy density of 3800 mAh/g. Because of its two valence charges, Mg has a specific volumetric capacity of 3833 mAh/cc, higher than that of the lithium metal (2046 mAg/cc). Compared with lithium, magnesium metal is cheap and naturally abundant.
However, although rechargeable magnesium batteries have been studied for more than a decade, they are still facing several obstacles. First, the reactivity of magnesium toward electrolytes results in passivation of the magnesium surface. Unlike lithium ions, which can move through a solid electrolyte interface containing inorganic lithium salts (e.g., lithium carbonate and lithium fluoride), magnesium ions cannot pass such passivated films. Second, there is a need for safer and more efficient magnesium electrolytes.
Ether solutions of Grignard reagents allow reversible magnesium deposition/dissolution with a high coulombic efficiency. However, they have fairly low anodic stability with an electrochemical window less than 1.8 V (L. P. Lossius, et al., Electrochim. Acta, 41 (1996) 445-447). Typical Grignard reagents include those denoted by the formula RMgCl (where R=methyl, ethyl, butyl). By introducing a Lewis acid such as AlCl3 or AlCl2Et to complex with the Grignard reagent, the electrochemical window of the resulting magnesium-Al electrolyte can be substantially increased. The current state-of-the-art intercalation electrode is Mo6S8, which is conventionally used in a THF solution of a Grignard reagent, Mg(AlCl2EtBu)2, to construct a rechargeable magnesium battery. The optimized Mg(AlCl2EtBu)2 electrolyte has an improved electrochemical stability up to 2.4 V vs Mg/Mg2+ (D. Aurbach, et al., Nature, 407 (2000) 724-727). However, despite the 100% efficiency of deposition/dissolution toward the magnesium electrode, the Mg(AlCl2EtBu)2 electrolyte is highly flammable and has a relatively low solubility in THF solution. The large molecular weight of Mg(AlCl2EtBu)2 also makes it less attractive as an electrolyte.